1. Field of the Invention
The invention relates to a working appliance, in particular a tamper for soil compaction or a hammer, with a tamping or beating working mass driven linearly back and forth, via a crank mechanism and a spring assembly, by a motor belonging to the upper mass.
2. Discussion of the Related Art
Known tampers of this type are designed in such a way that the upper mass comprises approximately two thirds and the beating working mass or lower mass one third of the entire tamper mass, whilst the distances covered in each case by the upper mass and the working mass are in inverse ratio to one another. In this case, the movement of the upper mass is of the order of magnitude of 25 to 30 mm.
This movement of the upper mass at a frequency of 10-11 Hz has adverse effects, because these vibrations are transmitted to the body of the person controlling the working appliance via a control handle, in particular to the person""s hand and arm.
Although this transmission of vibrations to the body can for the most part be damped by the skilful attachment of rubber elements, there are nevertheless also high loads on the mounted drive motor, regardless of its design.
This problem also arises when the working appliance is mounted on other appliances or vehicles which may be seriously damaged by the vibrations occurring.
The output of the tamping system depends largely on the upper mass, since too great a working mass or too high a speed of the working mass moves the upper mass excessively and aggravates the problems described above.
These harmful effects can be partly inhibited by increasing the upper mass, but in this case the overall weight of the tamper is increased and the power consumption of the drive motor consequently rises.
DE-A 1 925 870 discloses a tamping appliance, in which a tamping foot can be driven, via a double crank mechanism, by a motor belonging to an upper mass. The drive power is distributed via gearwheels to two intermeshing crank disks which, via associated connecting rods, in each case drive rods connected to the tamping foot. Centrifugal weights are fastened in each case to the oppositely rotating crank disks, in such a way that the horizontal components of the centrifugal forces cancel one another, whilst the vertical components are added together and counteract the vibration acting on the tamper housing due to the vibration of the tamping foot. The arrangement described has to have a bulky design due to the provision of two rods connecting the upper mass to the tamping foot and due to the meshing crank disks. The overall weight of the tamper is considerable.
The object on which the invention is based is, without any appreciable increase in the overall mass, to achieve substantial stabilization of the upper mass, whilst ensuring that the working appliance has a compact design.
In accordance with the invention, this object is achieved by providing a working appliance having a countermass that counteracts vibrations induced by operation of the working mass. The countermass, which can be driven in rotation by the motor that drives the working mass, includes first and second centrifugal weights that rotate in opposite directions to one another. The first centrifugal weight which may be provided on the crank mechanism itself, rotates about an axis of rotation that is coaxial with the crank mechanism. The second centrifugal weight rotates about an axis of rotation that is at least essentially coaxial to the axis of rotation of the crank mechanism. However, the axis of rotation of the second centrifugal weight may be offset in the direction of working mass reciprocation from the axis of rotation of the crank mechanism and the first centrifugal weight.
The working appliance is defined in that the second centrifugal weight is arranged rotatably about an axis of rotation arranged essentially coaxially to the axis of rotation of the crank mechanism. The first centrifugal weight is coaxial with or seated directly on the axis of rotation of the crank mechanism, and the second centrifugal weight is arranged behind the first centrifugal weight and has approximately the same flywheel moment as the first centrifugal weight. The second centrifugal weight is driven in the opposite direction to the first centrifugal weight about an axis of rotation that is offset somewhat relative to the axis of rotation of the crank mechanism and parallel to the direction of movement of the working mass.
In another embodiment of the invention the countermass can be moved linearly back and forth, parallel to the direction of movement of the working mass, with a phase shift unequal to 180xc2x0 in relation to the movement of the working mass.
The upper mass is pressed upward by a crank mechanism at the moment when the crank mechanism presses the working mass and, therefore, the tamping foot downward via the connecting rod of said crank mechanism, a guide piston and a spring assembly. The result of the spring assembly is that, during the downward movement of the guide piston, the springs are first tensioned, so as to absorb energy, whereupon, with a delay caused thereby, they subsequently discharge the stored energy again for the downward load on the tamping foot. This delay must be taken into account when the movement of the countermass is to be coordinated as accurately as possible with the movement of the working mass. When the working mass is drawn upward again by the crank pin of the crank mechanism, the upper mass is moved downward.
In a further advantageous refinement, therefore, the movement of the spring assembly end connected to the crank mechanism and the movement of the countermass are offset relative to one another with respect to the crank angle by 180xc2x0 minus a phase shift derived from the design parameters of the spring assembly.
By means of the crank mechanism, an oscillating movement of the upper mass relative to the working mass is built up, and only when the crank connected to the spring assembly goes past its vertex facing the spring assembly is the energy stored until then in the spring assembly released as tamping or beating energy, so that the countermovement of the countermass for damping the movement of the upper mass is required only at this moment, that is to say the movement of the countermass into its position furthest away from the working mass is to take place only when the crank connected to the spring assembly has passed through this vertex facing the spring assembly. This is achieved by means of the phase shift described above, which, in practice, must be coordinated at least approximately with the design parameters.
According to an expedient refinement, the countermass is guided on the upper mass in parallel with the direction of movement of the working mass. In this case, in an advantageous embodiment, the countermass is driven by a compensating eccentric on the crank mechanism, specifically, for example, via a connecting rod. According to another expedient refinement, the connection between the countermass and the compensating eccentric may be designed as a slider crank.
According to another expedient variant, the countermass consists of two part masses arranged in each case on one side of the crank mechanism and the other at approximately the same height with respect to the axis of rotation of the crank mechanism, and each part mass is driven by an eccentric pin on an eccentric disk assigned to the part mass and rotatably coupled to the crank mechanism, the connection between the eccentric pin and the associated part mass being designed in each case as a slider crank.
In a further advantageous variant, the countermass consists of flyweights which are mounted on the upper mass rotatably about mutually parallel axes and are driven in rotation in opposite directions by the crank mechanism and the flywheel moment and mutual phase relationship of which are organized in such a way that they generate an oscillation directed in counteraction to the working mass, a further refinement being that the countermass consists of two centrifugal weights which are arranged symmetrically to the direction of movement of the working mass next to one another at about the same height in this direction and which are directly coupled to one another so as to be rotatable in opposite directions and are driven by the crank mechanism.
In another expedient embodiment for avoiding lateral forces, the countermass consists of a first centrifugal weight seated directly on the shaft of the crank mechanism and of two second centrifugal weights arranged symmetrically to the direction of movement of the working mass next to one another at about the same height in this direction and having flywheel moments equal to one another, such centrifugal weights being driven rotatably by the crank mechanism in the opposite direction to the first centrifugal weight and their flywheel moment in each case being about half the flywheel moment of the first centrifugal weight.
In yet another variant, the countermass consists of a first centrifugal weight seated directly on the axis of rotation of the crank mechanism and of a second centrifugal weight which is arranged behind said first centrifugal weight and which has about the same flywheel moment as the first centrifugal weight and which is driven in the opposite direction to the first centrifugal weight about an axis of rotation offset somewhat, parallel to the direction of movement of the working mass, relative to the axis of rotation of the crank mechanism.